Research On Optimization Of Fragmentation Management In Flash Storage System

In the era of big data,flash memory has gradually replaced disk as the mainstream storage device due to its superior performance.However,in flash storage system,the logstructured writing scheme of flash file system and the address mapping function of flash transfer layer make the data blocks and free blocks discretely distributed in file system and flash,which brings serious fragmentation problem.Fragmentation management is divided into fragmentation avoidance and defragmentation.These existing schemes of fragmentation avoidance are mainly oriented to disk file systems and not suitable for flash file systems.Besides,defragmentation introduces a lot of overhead,which reduces the life of flash severely.Separated temporary data technique is proposed to reduce the logical layer fragmentation in flash file system.In flash file system,the data in temporary is the main reason generating logical fragmentation.Temporary data,separated from other data with different writing feature,are log-structured written to separate logical log segment,so that logical fragmentation in temporary files and other files are reduced.The logical layer fragmentation is alleviated.Segmented address-mapping technique is proposed to reduce the physical layer fragmentation in flash transfer layer.File system submits marked block request,and flash transfer layer constructs a request queue for the data belonging to the same logical log segment,so that the continuous logical blocks are written to the continuous physical pages,largely aligning logical address space and the physical address space.The physical layer fragmentation is alleviated.Experiments are completed at different fragmentation levels.Separated temporary data technique can improve sequential read performance by up to 29.0%.Segmented addressmapping technique can improve sequential read performance by up to 45.7%;The combination can improve sequential read performance by up to 58.4%.The experimental results show that the optimization in physical layer gains more than that in logical layer,and the higher the system capacity utilization is,the more serious the fragmentation problem is,the more improvement optimization brings.